The present invention relates to a novel method and apparatus for the simultaneous transmission of audio and video information over a medium with restricted bandwidth, and more particularly, it relates to the transmission of digitized video images in part of the band of such media, while voice or other audio information is simultaneously transmitted through another portion of the band.
It is well known that standard NTSC video images can be sent on coaxial cable, optical fiber, earth satellite, and microwave links. Such links have pass bands in the megahertz to gigahertz range, and so are capable of transmitting a 4.5- or 6-MHz signal required for NTSC video (or even greater bandwidths for SECAM and HDTV) and/or audio and video transmission in real time.
Such transmission is relatively costly because of the bandwidth requirements and the concomitant need for expensive cabling. For various historical and practical reasons, the conventional telephone "loop" or connection from a local subscriber to a central office is by means of a twisted-wire pair. The telephone system deliberately utilizes an overall band width of about 4 kHz, and in practice the pass band utilized encompasses from about 100 to about 3500 hertz. This is considered adequate, not only to convey the meaning of speech, but also to provide at least some of the particular timbre or character of a speaker's voice so that individual voices can frequently be recognized.
Amateur radio has available a relatively limited bandwidth, but over the years radio amateurs have successfully transmitted video images with a technique called "slow-scan television". A number of these techniques are described, for example, in Ingram, "The Complete Handbook of Slow-Scan TV", Tab Books, 1977. These techniques have enabled the transmission of images over distances of three-quarters of the earth's circumference, without satellite or forward-scatter techniques. Moreover, the simultaneous transmission of video and audio signals through ordinary telephone lines has been shown in U.S. Pat. No. 3,873,771. The method of the patent utilizes an FM slow-scan video signal and an AM single side band signal occupying the bandwidth of 100 to 1000 hertz. This narrowed audio signal can sacrifice some of the clarity of the voice and some of the distinctive character and timbre otherwise perceived at the receiver. An improvement of this patent in U.S. Pat. No. 3,851,095 describes the use of an image storage tube. Such apparatus is available, but the cost is relatively high, so that it cannot readily be furnished to a large number of consumers.
U.S. Pat. No. 4,503,468 shows a pair of linked terminals which are capable of generating images and altering such images at local and remote input devices. U.S. Pat. No. 4,360,827 shows a method and system for establishing interactive audio and video conferences. The audio signal is sent over ordinary telephone lines, while the video signal is transmitted via satellite. U.S. Pat. No. 4,349,701 relates to a television receiver which has telephone network access means. The receiver provides for viewing a television picture while utilizing a talk channel for carrying out a two-way telephone conversation. U.S. Pat. No. 4,338,492 also shows a similar system.
U.S. Pat. No. 4,494,144 shows a reduced bandwidth video transmission system which is designed to provide a good video signal of selected portions of a bandwidth. U.S. Pat. No. 4,485,400 shows apparatus and a method for transmitting video data over telephone lines, and is said to be particularly useful for transmitting and receiving images of persons speaking to each other. The method utilizes a television camera with slow scanning and image retaining means for displaying the image of a person having a similar unit.
U.S. Pat. No. 4,388,489 shows a so-called video conversational data communication network to permit conversational video textual data transmission. U.S. Pat. No. 4,386,365 shows a wired broadcast system for television, so as to provide visual communication through a frequency changer.